The solid Earth - Geochemistry: An Introduction - page 216

thick beneath the oceans and at least twice as thick beneath the continents. They move
apart along mid-ocean ridges and converge along subduction zones (marked by the deepest
ocean floors, the trenches), where one plate slides under another.
Plate tectonics is not a separate mechanism from mantle convection, but is merely its
surface expression. The mantle is not a molten medium: like ice flowing in glaciers, the
mantle is a solid that is nonetheless deformable when worked over geological time scales.
Convection is a generalized movement of the mantle maintained by density inversions
(heavy above light) brought about by thermal contrasts within the Earth: a material is
lighter when hot than when cold. But also, hot material deforms more easily than cold
material, so that radioactive heating not only makes the mantle hotter and lighter but softer,
which facilitates convective movements. The temperature dependence of viscosity there-
fore acts as a thermal regulator for the thermal regime of the mantle. Hot mantle material
pours out from under the mid-ocean ridges, while cold lithospheric plates are injected deep
into the mantle by subduction. Mantle convection therefore has the effect of extracting heat
by replacing deep hot mantle by material that was cooled next to the surface. Heat is lost
through the surface ( Fig. 11.2 ) by conduction through the lithosphere and by advection of
magmas at mid-ocean ridges and other volcanic sites. The potential energy of sinking plates
is converted into heat (dissipation) at subduction zones, where the oceanic lithospheric
plates bend. Since thermal conduction through the lithospheric plates is a rather inefficient
cooling mechanism, it is not exaggerated to say that the mantle cools from below thanks to
the subduction of young cold plates.
Density inversions are maintained by heat from radioactive elements (U, Th, K) con-
tained in the mantle but also by the heat released from the core. In this case, fluid mechanics
teaches us that convection in a medium heated from the bottom is unstable, with irregular
spurts of hot, less dense material rising rapidly to the surface: these are hot spots or vol-
canic plumes that begin catastrophically at the core-mantle boundary. The heads of these
instabilities produce gigantic eruptions covering areas larger than Texas with lava in com-
paratively short geological time spans (about a million years) and may break up whole
continents. The separation of North America and Europe or of Antarctica and Australia is
attributed to this process. The residual plume tail may remain active for tens of millions of
years and, as plates move across the surface, it may form strings of volcanoes thousands
of kilometers long, such as those of Hawaii. Where these plates are not continental, cool-
ing makes them progressively denser and they sink into the mantle as planar structures at
subduction zones. The cold parts constantly drag the remainder of the plate down into the
mantle where it tears apart at its thinnest points. This sinking is offset by the formation of
young lithosphere along the scars, the mid-ocean ridges, rather like a tablecloth slipping
over the edge of the table, from the center of which it is being constantly woven and fed
out. The subduction zones are the downgoing limbs of mantle convection, whereas mate-
rial rises from the deep mostly as part of the general circulation or as narrow upwellings
known as plumes.
The continents are formed by relatively light felsic material and behave like corks fixed
on top of the lithospheric plates. Mountain ranges form when two continental “corks”
collide: a fraction of the continental rocks may then be carried down to very great
depths (up to 200 km). There, they are subjected to very high temperatures and pressures